membrane structure and...
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PowerPoint® Lecture Presentations for
BiologyEighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Chapter 7
Membrane Structure and
Function
Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
• The plasma membrane is the boundary that separates the
living cell from its surroundings
• The plasma membrane exhibits selective permeability,
allowing some substances to cross it more easily than
others
• Phospholipids are the most abundant lipid in the plasma
membrane
• Phospholipids are amphipathic molecules, containing
hydrophobic and hydrophilic regions
• The fluid mosaic model states that a membrane is a fluid
structure with a “mosaic” of various proteins embedded in it
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Membrane Models
• Membranes have been chemically analyzed and found to be made of
proteins and lipids
• Scientists studying the plasma membrane reasoned that it must be a
phospholipid bilayer
• In 1972, J. Singer and G. Nicolson proposed that the membrane is a
mosaic of proteins dispersed within the bilayer, with only the
hydrophilic regions exposed to water
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The Fluidity of Membranes
• Phospholipids in the plasma membrane can
move within the bilayer
• Most of the lipids, and some proteins, drift
laterally
• Rarely does a molecule flip-flop transversely
across the membrane
• As temperatures cool, membranes switch from
a fluid state to a solid state
• The temperature at which a membrane
solidifies depends on the types of lipids
• Membranes rich in unsaturated fatty acids are
more fluid that those rich in saturated fatty
acids
• Membranes must be fluid to work properly;
they are usually about as fluid as salad oil
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• The steroid cholesterol has different effects on membrane fluidity at
different temperatures
• At warm temperatures (such as 37°C), cholesterol restrains
movement of phospholipids
• At cool temperatures, it maintains fluidity by preventing tight packing
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Membrane Proteins and Their Functions
• A membrane is a collage of different proteins embedded in the fluid
matrix of the lipid bilayer
• Proteins determine most of the membrane’s specific functions
• Peripheral proteins are bound to the surface of the membrane
• Integral proteins penetrate the hydrophobic core
• Integral proteins that span the membrane are called transmembrane
proteins
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• Six major functions of
membrane proteins:
– Transport
– Enzymatic activity
– Signal transduction
– Cell-cell recognition
– Intercellular joining
– Attachment to the
cytoskeleton and
extracellular matrix
(ECM)
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The Role of Membrane Carbohydrates in Cell-Cell Recognition
• Cells recognize each other by binding to surface molecules, often
carbohydrates, on the plasma membrane
• Membrane carbohydrates may be covalently bonded to lipids (forming
glycolipids) or more commonly to proteins (forming glycoproteins)
• Carbohydrates on the external side of the plasma membrane vary
among species, individuals, and even cell types in an individual
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 7.2: Membrane structure results in selective permeability
• A cell must exchange materials with its surroundings, a process controlled by
the plasma membrane
– Plasma membranes are selectively permeable, regulating the cell’s molecular
traffic
– Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the
lipid bilayer and pass through the membrane rapidly
– Polar molecules, such as sugars, do not cross the membrane easily
• Transport proteins allow passage of hydrophilic substances across the
membrane
– Some transport proteins, called channel proteins, have a hydrophilic
channel that certain molecules or ions can use as a tunnel
– Channel proteins called aquaporins facilitate the passage of water
• Other transport proteins, called carrier proteins, bind to molecules and change
shape to shuttle them across the membrane
• A transport protein is specific for the substance it moves
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Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment
• Diffusion is the tendency for molecules to
spread out evenly into the available space
• Although each molecule moves randomly,
diffusion of a population of molecules may
exhibit a net movement in one direction
• At dynamic equilibrium, as many molecules
cross one way as cross in the other
direction
• Substances diffuse down their
concentration gradient, the difference in
concentration of a substance from one area
to another
• No work must be done to move substances
down the concentration gradient
• The diffusion of a substance across a
biological membrane is passive transport
because it requires no energy from the cell
to make it happen
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Effects of Osmosis on Water Balance
• Osmosis is the
diffusion of water
across a selectively
permeable membrane
• Water diffuses across
a membrane from the
region of lower solute
concentration to the
region of higher solute
concentration
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Water Balance of Cells Without Walls
• Tonicity is the ability of a
solution to cause a cell to
gain or lose water
• Isotonic solution: Solute
concentration is the same as
that inside the cell; no net
water movement across the
plasma membrane
• Hypertonic solution: Solute
concentration is greater than
that inside the cell; cell loses
water
• Hypotonic solution: Solute
concentration is less than
that inside the cell; cell gains
water
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• Hypertonic or hypotonic
environments create
osmotic problems for
organisms
• Osmoregulation, the
control of water balance,
is a necessary
adaptation for life in
such environments
• The protist Paramecium,
which is hypertonic to its
pond water environment,
has a contractile vacuole
that acts as a pump
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Water Balance of Cells with Walls
• Cell walls help maintain water
balance
• A plant cell in a hypotonic solution
swells until the wall opposes uptake;
the cell is now turgid (firm)
• If a plant cell and its surroundings
are isotonic, there is no net
movement of water into the cell; the
cell becomes flaccid (limp), and the
plant may wilt
• In a hypertonic environment, plant
cells lose water; eventually, the
membrane pulls away from the wall,
a usually lethal effect called
plasmolysis
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Facilitated Diffusion: Passive Transport Aided by Proteins
• In facilitated diffusion, transport
proteins speed the passive movement
of molecules across the plasma
membrane
• Channel proteins provide corridors that
allow a specific molecule or ion to
cross the membrane
• Channel proteins include
– Aquaporins, for facilitated
diffusion of water
– Ion channels that open or close
in response to a stimulus (gated
channels)
• Carrier proteins undergo a subtle
change in shape that translocates
the solute-binding site across the
membrane
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Concept 7.4: Active transport uses energy to move solutes against their gradients
• Facilitated diffusion is still passive because the solute moves down its
concentration gradient
• Some transport proteins, however, can move solutes against their
concentration gradients
• Active transport moves substances against their concentration gradient
• Active transport requires energy, usually in the form of ATP
• Active transport is performed by specific proteins embedded in the
membranes
• Active transport allows cells to maintain concentration gradients that differ
from their surroundings
• The sodium-potassium pump is one type of active transport system
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2
EXTRACELLULAR
FLUID[Na+] high
[K+] low
[Na+] low
[K+] high
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
ATP
ADP
P
Na+
Na+
Na+
P
3
6 5 4
PP
1
Fig. 7-16-7
Fig. 7-17Passive transport
Diffusion Facilitated diffusion
Active transport
ATP
Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
• Small molecules and water enter or leave the cell through the lipid
bilayer or by transport proteins
• Large molecules, such as polysaccharides and proteins, cross the
membrane in bulk via vesicles
• Bulk transport requires energy
– In exocytosis, transport vesicles migrate to the membrane, fuse with it,
and release their contents
– Many secretory cells use exocytosis to export their products
• In endocytosis, the cell takes in macromolecules by forming vesicles
from the plasma membrane
• Endocytosis is a reversal of exocytosis, involving different proteins
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• In phagocytosis a cell engulfs a
particle in a vacuole
• The vacuole fuses with a
lysosome to digest the particle
• In pinocytosis, molecules are
taken up when extracellular fluid
is “gulped” into tiny vesicles
• In receptor-mediated
endocytosis, binding of ligands
to receptors triggers vesicle
formation
– A ligand is any molecule that
binds specifically to a receptor
site of another molecule
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3 Types of endocytosis